Plasticizers are incorporated into a resin (usually a plastic or elastomer) to increase the flexibility, workability, or distensibility of the resin. The largest use of plasticizers is in the production of “plasticized” or flexible polyvinyl chloride (PVC) products. Typical uses of plasticized PVC include films, sheets, tubing, coated fabrics, wire and cable insulation and jacketing, flooring materials such as vinyl sheet flooring or vinyl floor tiles, adhesives, sealants, inks, and medical products such as blood bags and tubing, and the like.
Other polymer systems that use small amounts of plasticizers include polyvinyl butyral, acrylic polymers, poly(vinyldiene chloride), nylon, polyolefins, and certain fluoroplastics. Plasticizers can also be used with rubber (although often these materials fall under the definition of extenders for rubber rather than plasticizers). A listing of the major plasticizers and their compatibilities with different polymer systems is provided in “Plasticizers,” A. D. Godwin, in Applied Polymer Science 21st Century, edited by C. D. Craver and C. E. Carraher, Elsevier (2000); pp. 157–175.
Plasticizers can be characterized on the basis of their chemical structure. The most important chemical class of plasticizers are phthalic acid esters, which accounted for about 85% worldwide of PVC plasticizer usage in 2002. Two other important chemical classes are adipic acid esters, and trimellitic acid esters. Di- and tri-esters of these aforementioned acids, having a molecular weight range from about 300 to 600, typically offer a balance of solvency and compatibility with the resin, yielding plasticized materials with useful properties and good aging abilities.
Trimellitate esters are used as plasticizers in those applications where greater permanence is required. These esters are similar in structure to the phthalic acid esters, except for having a third ester functionality on the aromatic ring. Trimellitate esters provide for greater permanence primarily from reduced volatility losses but also offering reduced losses attributed to lower migration rates into other materials. Plasticized PVC electrical wire insulation prepared from either tri-2-ethylhexyl trimellitate (TOTM) or the even more permanent plasticizer triisononyl trimellitate (TINTM) will survive longer periods of high temperature service versus those products prepared from more volatile phthalate plasticizers currently available. However, the trimellitate esters are generally much more expensive, typically costing 2–3 times that of the phthalate esters such as DEHP (di-2-ethylhexyl phthalate) or DINP (diisononyl phthalate) and yield more expensive plasticized PVC electrical wire insulation. PVC formulations using trimellitate plasticizers are also more difficult to process when compared with PVC formulations that use only phthalate esters as plasticizers.
Plasticizer selection for electrical wire insulation is dependent upon the performance specifications of the insulation material and the jacketing. Performance specifications and tests such as accelerated aging tests (at various temperatures), and the like are well known in this art and are described by UL (Underwriters Laboratory) methods, e.g., UL 83. For example, for those products designed for extended periods of use at 90° C. or 105° C., often evaluated in accelerated aging studies for seven (7) days at 136° C., will contain primarily the more costly trimellitate plasticizers. A typical formulation for this 90° C. or 105° C. rated product is shown in Table 1, column A. (European designations are different from those used in the United States. For instance, 105° C. designations according to VDE Specification Code 0207 are YI 8 and YM 4).
On the other hand, flexible PVC insulation designed for extended periods of use at 60° C., characterized by accelerated oven aging testing at 80° C. or 100° C., can be prepared from less costly plasticizers such as DINP or DEHP. A typical formulation for this 60° C. rated product is shown in Table 1, column B.
Flexible PVC compounds prepared with trimellitate esters such as TOTM or TINTM generally exceed the minimum retained properties after aging specification for 90° C. or 105° C. electrified wire insulation compounds, such as those required to meet the 105° C. Class 12 (UL62), 105° C. Appliance (UL758), NM-B 90° C. building wire (Romex®, non-metallic sheathed cable, PVC jacket), or THHN 90° C. building wire (thermoplastic PVC insulation, high heat resistant, 90° C. rating, dry or damp, nylon jacket), while those products prepared with only the lower cost phthalate esters fail.
However, it is a common practice to partially substitute some of the expensive trimellitate esters with less expensive, higher molecular weight phthalate esters. As the concentration of phthalate ester in the plasticizer system increases, performance in the accelerated aging test will decrease, but there is enough flexibility in this formulating to offer a measurable cost savings while still meeting the product performance requirements.
An example of this use of triimellitate plasticizer blended with a heavier molecular plasticizer is the formulation described by L. G. Krauskopf, Handbook of PVC Formulations,” edited by E. J. Wickson, “Monomeric Plasticizers,” 1993, John Wiley & Sons, page 201, which describes for UL method 83 THHN applications, an insulation material prepared using the formulation shown in Table 1, column C. “UDP” is undecyl dodecyl phthalate (Jayflex™ UDP, available commercially, as are all Jayflex™ plasticizers cited herein, from ExxonMobil Chemical Company, Baytown, Tex.). The stabilizer used is Dythal™ lead stabilizer, available commercially as a phthalate or sulfate salt. According to this reference, the formulation exhibited 72% retained elongation after aging for seven (7) days at 136° C., exceeding the minimum specification of 65% retained elongation.
TABLE 1ABC 100 kg PVC resin 100 kg PVC100 kg PVC  45 kg TOTM 25 kg TINTM  60 kg DINP 25 kg UDP  30 kg CaCO3 or clay  50 kg calcium 12 kgcarbonatecalcined clay  6 kg lead stabilizer  5 kg lead stabilizer 6 kg leadstabilizer  6 kg antimony trioxide  6 kg antimony 6 kg antimonytrioxidetrioxide0.25 kg stearic acid0.25 kg stearic acid 0.2 kg stearicacid
Other phthalates commonly blended with trimellitate esters to reduce costs while exceeding specification are diundecyl phthalate (DUP, available commercially as Jayflex™ L11P), and ditridecyl phthalate (DTDP, available commercially as Jayflex™ DTDP).
The blending of phthalate esters with trimellitate esters to make PVC insulation or jacketing PVC compounds also contributes to improved processability by reducing the melt viscosity of the flexible PVC compound. In the preparations of PVC compounds for high temperature applications, it is preferable to use as much phthalate ester as possible in the plasticizer mixture, to help reduce costs and improve processability.
However, because of the higher volatility of the phthalate esters, there are practical limitations in the type and level of phthalate esters which cannot be exceeded, for at higher phthalate levels the products begin to fail the retained tensile properties listed in the various specifications. For example, current blends of TOTM with DUP are limited to about 40 wt. % DUP as the maximum because at higher levels, product failures start to occur in retained elongation and retained tensile properties after accelerated aging, resulting in a brittle product. For this reason it is common to find commercial products with only 20 wt. % to 40 wt. % DUP in blends with TOTM to avoid product failures. As Jayflex™ DTDP has slightly lower volatility than DUP, it can be used in higher concentrations. However, it is still limited to about a 60 wt. % concentration in blends with the heavier molecular weight TINTM plasticizer, with more typical concentrations being around 50 wt. % DTDP in TINTM.
In addition to the aforementioned problems, there is also a need for alternative plasticisers to avoid problems with migration out of the plasticized material. Phthalate esters with reduced volatility facilitate their usage at higher concentrations in trimellitate blends, yielding additional cost savings and improvements in processability while still exceeding the performance specifications in the accelerated aging testings.
Important properties of a plasticizer include without limitation high plasticizing efficiency, excellent compatibility with the resin, excellent processability, excellent oxidative stability, and low volatility. Usually, when changes are made to improve one of these properties, some other important property is adversely affected. For example, an increase in alcohol molecular weight tends to reduce volatility at the expense of plasticizing efficiency. In addition, as the molecular weight of the phthalate or trimellitate ester plasticizer increases, its compatability with PVC decreases, eventually resulting in a less desirable flexible PVC product with limited potential.
The range of alcohols useful in esterification for plasticizers is generally limited from about C4 to about C13 monohydridic alcohols. It is known that the specific alcohols from which the esters are made influences the performance properties, e.g., the size and structure of the alkyl group helps determine the volatility and gellation temperature of the plasticisers and is therefore chosen according to the application in which the plasticized polyvinyl chloride is to be used. The alcohols from which the plasticisers esters are made are generally obtained by either olefin oligomerization followed by hydroformylation or by hydroformylation of olefins to form aldehydes followed by aldehyde dimerization, generally by an aldol reaction. The alkyl groups of the esters therefore vary in size and structure according to the process used to produce the alcohols.
U.S. Pat. No. 2,842,514 describes using alcohol mixtures obtained by the reaction of aldehydes obtained by the “Oxo” process, wherein an olefin feed is oxonated with carbon monoxide and hydrogen at elevated temperature and pressure in the presence of a cobalt catalyst. Particularly effective plasticizers are said to derive from certain polyhydric alcohols derived from the Oxo process esterified with C5–C7 saturated aliphatic acids.
U.S. Pat. No. 4,426,542 describes a process in which mixed butenes are converted to a C10 plasticizer alcohol comprised of at least about 80–90% 2-propyl-heptanol by an oxo reaction. It is taught, for instance, that 2-propylheptanol is a well-suited plasticizer alcohol whereas 2-propyl-4-methyl-hexanol has much poorer properties. This patent teaches to moderate the temperature in the hydroformylation reaction to achieve a higher ratio of normal versus branched product, the former being more desirable.
U.S. Pat. No. 4,806,425 describes the use of electrical wiring products based on dialkyl phthalate esters having at least 11 carbon atoms in the alkyl groups and having a “higher than normal amount of antioxidant.” Adding more antioxidant is not a preferred solution to the problem of getting higher phthalate blends because increased antioxidant can lead to decreases in volume resistively and can cause problems with color stability.
U.S. Pat. Nos. 5,189,105 and 5,468,419 are directed to obtaining a C9 plasticizer alcohols with good cold resistance and electrical insulating properties, obtained by hydroformylating octenes derived from butene dimerization. The desired product is generally obtained by taking selected portions of the alcohol mixture obtained after conventional hydroformylation.
U.S. Pat. Nos. 5,268,514; 5,369,162; 5,382,716; 5,462,986; and 5,463,147 describe mixtures of isomeric decyl alcohols obtained by hydroformylation of 1- and 2-butene containing mixtures to obtained linear or “slightly branched chain alcohols.” Phthalate esters obtained using these decyl alcohols are taught to be useful in PVC compositions, having particularly good “cold resistance.”
U.S. Pat. No. 5,414,160 is concerned with an organonickel catalyst system capable of improving the yield and selectivity of octenes having a low degree of branching. The average degree of branching of the octenes is from 0.85 to 1.15. Plasticizer C9 alcohols are obtained after hydroformylation of the thus-described octenes.
U.S. Pat. Nos. 5,516,948; 5,583,250; and 5,661,204 describe mixture of isomeric decyl alcohols obtained by oligomerization of propylene in the presence of deactivated zeolites as catalyst, followed by separation of the C9 olefins from the oligomer mixture, then hydroformylation of the C9 olefins to C10 aldehydes, followed by hydrogenation to the corresponding alcohols. The mixtures are esterified with phthalic acid or anhydride. The alcohol product claimed in the U.S. '250 patent is at least 80% linear.
U.S. Pat. No. 6,355,711 describe a plasticizer ester prepared by esterifying an acid or anhydride with C7–C11 oxo alcohols prepared by hydroformylating C6–C10 olefins having at least 50% methyl branching at the beta carbon. Examples of this invention are C9 phthalate esters obtained from the C9 alcohol produced by the hydroformylation of 2-methyl heptene-1 or 2-methyl heptene-2. They are described by the patent as being useful particularly in the manufacture of PVC automotive interior trim applications, and in electrical wire jacketing compounds, however, the plasticizers discussed are too volatile for certain high-temperature applications, e.g., high temperature electrical wiring.
U.S. Pat. No. 6,437,170 relates to a mixture of isomeric nonanol diesters of adipic or phthalic acid, wherein the alcohol component of the diesters are formed from an isomeric nonanol mixture. The composition is characterized by a specific ratio of methylene and methylidene groups to methyl groups in the isononyl radical, as measured by 1H NMR spectra, obtainable preferably by butene dimerization using a nickel oxide catalyst followed by hydroformylation.
It is known that as the linearity of the alcohol used to make the phthalate ester increases, certain predictable events occur. One may expect reduced plasticizer volatility, improved plasticizer efficiency towards making PVC flexible, improved low temperature and flexibility, and sometimes improved processability, the latter characteristic being often a combination of plasticizer solvency and plasticizer viscosity. However, as the linearity of a plasticizer increases, its compatibility with PVC decreases, where “compatibility” is used to reference a usable product with no or slight exudation under stress. For phthalate esters based on C13 alcohols heretofore available, compatibility with PVC is thought to decrease as the linearity increase. One reference, Alan S. Wilson, Plasticisers, University Press (1995), p. 137 (FIG. 4.6), indicates that for a branching index of less than 20 (20% of the total carbons are branching carbons), phthalate esters based on C13 alcohols exhibit poor compatibility and poor processability. Commercially available phthalate esters such as Jayflex™ DTDP plasticizers are typically derived using a C12 olefin obtained from a SPA (solid phosphoric acid) unit, followed by hydroformylation in the Oxo process. Jayflex™ DTDP has an average of 3.2 branches per alcohol moiety and a branching index of about 25 (assuming all methyl branches and calculated using the average carbon number of 12.7; some ethyl branches are present). SPA units, also known as polygas units, are well-known in the art, as discussed for instance in U.S. Pat. Nos. 6,284,938; 6,080,903; 6,072,093; 6,025,533; 5,990,367; 5,895,830; 5,856,604; 5,847,252; and 5,081,086.
The present inventor has surprisingly discovered, however, that plasticizers based on esters having, as the alcohol moiety, less branched C13 alcohols provide for at least one of the properties of improved plasticized resin compatibility, improved processability of the resin/plasticizer mixture, lower volatility, and improved aged performance characteristics in articles formed therefrom without significantly effecting the other important properties of the plasticizer, and/or plasticizer/resin mixture, and/or final product.